The invention relates to a double wall underground storage tank with leak-detecting means. More particularly the invention is directed to a double wall tank with a integral reservoir mounted on the tank for the leak-detecting liquid.
In double wall tanks there is an annular space between the two walls and leak detecting liquid is provided in this space. Loss of the leak detecting liquid occurs when a leak develops either in the inner wall or the outer wall of the tank. The space between the walls of prior underground tanks is connected, as by a pipe, to an above-ground reservoir having a liquid level monitoring device and usually an alarm system associated with the monitoring device. Upon development of a leak, the liquid level in the reservoir drops, the monitoring device senses a change in the liquid level and the monitoring device sets off the warning alarm. This type of system requires considerable plumbing to connect the annular space with the external reservoir for the leak detecting liquid. It is not unusual for leaks to develop in the plumbing connecting the annular space to the reservoir and this can result in a false indication that there is a leak in the tank. Since the external reservoir is located above ground, the liquid in the reservoir can be subjected to conditions not experienced by the leak detecting liquid in the annular space between the inner and outer wall of the tank. Freezing and extremely hot temperatures can produce conditions that can effect the operation of the leak detecting system so that the system does not function properly. All of the possible indicators of a false leak in the underground storage tank provides a great deal of difficulty for the owner of the tank. Usually material stored in the tank can have a detrimental effects if it leaks from the tank. Accordingly, the owners of the tank are required to very carefully monitor the condition of the tank and the level of the leak detecting liquid in an attempt to determine if in fact there is actually a leak in the tank. From some of the possible failures of the leak detecting system it can be extremely difficult to determine if the leak is in the plumbing for the leak detecting system or actually in the tank. Accordingly, it might be necessary to remove the material from the tank so that the tank can be dug up and replaced to prevent any chance that liquid is leaking from the tank.
Recently integral reservoirs for the leak detecting fluid have been mounted on the outer wall of the double wall tank. These reservoirs are formed by a manway type fitting that is secured to the outer wall of the double wall tank and this fitting is in alignment with a corresponding hole in the outer wall of the tank. The lower end of the manway type fitting terminates at the outer wall of the tank and communicates with the annular space between the two tank walls. A metal cover is secured by suitable means to the flange of the manway type fitting that is used to form the reservoir. The normal leak detecting device extends through the metal lid and into the reservoir. This is an improvement over the prior external reservoirs but there are still problems because of the metal lid, the connection of the metal lid with the flange of the manway type fitting and the connections between the metal lid and the leak monitoring equipment that passes through the metal lid. Since the reservoir is integral with the tank, the reservoir is below ground and there can be serious corrosion problems associated with the metal lids. It is also very difficult to prevent leaks between the metal lid and the leak monitoring equipment. Accordingly, there is still the potential that false indications of a leak in the tank can be generated by the leak monitoring equipment due to failures associated with the manway type reservoir.
Accordingly, there is a need for an improved reservoir that can be mounted integrally on the double wall storage tank where the reservoir and all of the connections are made of a non-corrosive material that is substantially the same as material used to form the tank. It is also desirable to have a one-piece reservoir to simplify the construction of the reservoir and minimize the connections where leaks can occur in the reservoir.